1. Field of the Invention
This invention generally relates to an offset printing press and more particularly to such a printing press with a pair of blanket cylinders movably mounted by a throw off apparatus to enable separation from each other and from a pair of plate cylinders.
2. Description of the related art including information disclosed under 37 CFR 1.97-1.99
Referring to FIGS. 1A and 1B, shown is a schematic view of one end of a conventional, or prior art, offset press 10 including a pair of substantially identical plate cylinders 12A and 12B mounted for rotation about fixed axes of rotation 20A and 20B and which carry an engraved plate (not shown) of images to be printed on a web of paper 13 passing between a pair of blanket cylinders 14A and 14B. Each of the blanket cylinders 14A and 14B carry cylindrical blankets 14A' and 14B' respectively wrapped therearound. The images engraved on the plates of plate cylinders 12A and 12B are transferred as inked images to the blankets 14A' and 14B' of plate cylinder 14A and 14B The inked images carried by the blankets 14A' and 14B', in turn, are transferred to opposite sides of the paper web 13. For each of the plate cylinders 12A and 12B, there is an inking train of rollers to deliver ink to the plate cylinders 12A and 12B and a dampening train of rollers to deliver dampening liquid to the plate cylinders 12A and 12B which are conventional and therefore not shown. Those inking train rollers and dampening liquid train of rollers are in generally tandem rolling contact with each other and ultimately, at their ends, with the plate cylinders 12A and 12B and are relatively fixed and immovable during normal use of the press.
In FIG. 1A, the offset printing press 10 is shown in an operational position in which the plate cylinders 12A and 12B are in rolling transfer contact with the blankets 14A' and 14B' of the blanket cylinders 14A and 14B, respectively, and the paper web 13 is run through the contact nib between the pair of blanket cylinders 14A and 14B. During normal operation of the press 10, the blankets 14A' and 14B' must be periodically removed from the surface of the blanket cylinders 14A and 14B. When worn, the blankets 14A' and 14B' are removed and replaced. Accordingly, the pair of blanket cylinders 14A and 14B are movably mounted by means of a throw off apparatus for separation and creation of throw off gaps. Referring to FIG. 1B, a gap 41 is created between the blanket cylinders 14A and 14B and gaps 40A and 40B are created between the plate cylinders 12A and 12B and the blanket cylinders 14A and 14B, respective. The blankets 14A' and 14B' or other like blankets are mounted to and removed from through the gaps 40A, 40B and 41. Additionally, the creation of throw off gap 41 and throw off gaps 40A and 40B are needed to install and remove plates from the plate cylinders 12A and 12B and blankets 14A' and 14B' from the corresponding blanket cylinders 14A and 14B, respectively. Furthermore, a throw off gap 41 is needed to provide space between the blanket cylinders in case of a break in the web 13 during printing which consequently can lead to a web wrap-up around the blanket cylinders. The gaps provide room for the broken web to wrap and accumulate around the blanket cylinders thereby preventing possible damage to the blanket cylinders.
The throw off apparatus includes a drive linkage 15 pivotally connected to a peripheral arm of an eccentric member 44A which mounts axle 24A of the blanket cylinders 14A, and an interconnecting member 17 pivotally linking together another peripheral arm of the eccentric member 44A with a peripheral arm of another like eccentric member 44B mounting rotary axle 24B of the blanket cylinder 14B. The eccentric members 44A and 44B are mounted for rotation about fixedly mounted axes 25A and 25B, respectively, which are offset from the rotary axes 24A and 24B, respectively, of the blanket cylinders 14A and 14B.
In order to mount or dismount the blankets 14A' and 14B' when the printing press is nonoperational, the drive linkage 15 is moved by a conventional drive (not shown) to turn the eccentric members 44A and 44B in a counterclockwise direction from the operational position shown in FIG. 1A, as shown by arrows 42 and 47, FIG. 1A. This counterclockwise rotation causes the blanket cylinders 14A and 14B to move downwardly from the plate cylinders 12A and 12B and away from each other to create the necessary throw off gap 41 between the blanket cylinders and the pair of gaps 40A and 40B between blanket cylinders 12A and 12B and the pair of plate cylinders 12A and 12B, respectively.
As shown in FIGS. 1A and 1B, in the conventional printing press 10, both of the central axes of rotation 20A and 20B of the plate cylinders 12A and 12B are offset approximately thirty degrees from the blanket cylinders 14A and 14B and have axes substantially misaligned from lateral alignment with the central axes of rotation 24A and 24B of the blanket cylinders 14A and 14B. During operation, the rolling contact between the inking and dampening train of rollers and the plate cylinders 12A and 12B and between the plate cylinders 12A and 12B and movably mounted blanket cylinders 14A and 14B results in vibration between the blanket cylinders 14A and 14B and between the plate cylinders 12A and 12B and the blanket cylinders 14A and 14B, respectively. This vibration in a lateral direction generally transverse to the rotary axes results in momentary losses of proper printing pressure between the plate cylinders 12A and 12B and the blanket cylinders 14A and 14B. This vibration between the cylinders creates an uneven ink transfer from the plate cylinder to the blanket cylinder 14A and 14B. Disadvantageously, this relatively vibrational movement between the blanket cylinders 14A and 14B and the plate cylinders 12A and 12B and between the blanket cylinders 14A and 14B in the misaligned printing press 10 reduces the quality of the printed image on the paper web 13 passed between the blanket cylinders.
It is also known to have a printing press in which the axes of rotation of the blanket cylinders and the plate cylinders are substantially aligned. However, in these known printing presses at least one of the plate cylinders must be moved out of alignment in order to create a throw off gap between the blanket cylinders. Movement of a plate cylinder disadvantageously often leads to poor printing images. Moreover, in these known apparatuses a complicated and costly design is required to move the plate cylinders into and out of alignment.